1. Field of the Invention
The present invention relates to a phase difference film employable for a liquid crystal display, and a liquid crystal display provided with the phase difference film.
2. Description of Prior Art
A super twisted nematic liquid crystal display (hereinafter referred to as STN-LCD) has been recently utilized for dot-matrix type liquid crystal displays such as a word processor and a personal computer. The liquid crystal display has generally a structure that a liquid crystal cell is disposed between a pair of polarizing sheets. The STN-LCD uses a super twisted nematic liquid crystal showing a twisted angle of not less than 160 degrees. Such STN-LCD has an advantage of showing a high contrast on high multiplexing drive compared with the conventional twisted nematic liquid crystal display which can be utilized for the dot-matrix type liquid crystal displays.
A display screen of the STN-LCD shows an image inherently colored with the hue from green to yellowish red owing to elliptically polarized light transmitted through STN liquid crystal cell. This phenomenon may be hereinafter referred to as coloring. In order to avoid the coloring of the display screen, there is adopted a countermeasure of elimination of the coloring by changing the elliptically polarized light into linearly polarized light, which is accomplished by compensating a phase difference caused by birefringence of the STN liquid crystal. For example, there have been proposed a liquid crystal display using an additional liquid crystal cell which serves as an optically anisotropic body (that is, NTN mode-display) and a liquid crystal display using a uniaxial stretched polymer film (that is, FTN mode-display). The FTN mode-display utilizes a uniaxial stretched polymer film serving as optically anisotropic body instead of the liquid crystal cell. The former NTN mode-display shows excellent characteristics in black-and-white display. The liquid crystal cell for optically anisotropic body used in the NTN mode-display use is voluminous and a plurality of such cells are required, so that the NTN mode-display is larger in volume and weight, and higher in production cost than the FTN mode-display.
Japanese Patent Provisional Publication No. 63(1988) 189804 proposes, as a uniaxial stretched film for the FTN mode, use of a polycarbonate film which is uniaxially stretched so that a value of retardation measured by a polarizing microscope is set in the range of 200 to 350 nm or 475 to 625 nm. The retardation is defined by the product of birefringence (.DELTA.n) and thickness (d) of the film.
Japanese Patent Provisional Publication No. 63(1988). 167304 discloses a laminated film in which two or more uniaxial stretched films having birefringence are so laminated as to allow their main optic axes to intersect at right angles each other. In the case that two birefringence films (their retardation values are named R.sub.1 and R.sub.2, respectively) are laminated under the intersection at right angles, the retardation of the composite film (i.e., phase difference film) shows a retardation value of ".vertline.R.sub.1 -R.sub.2 .vertline.". Such film can be adjusted so that the value of the ".vertline.R.sub.1 -R.sub.2 .vertline." is set in the range of 90 to 180 nm, 200 to 350 nm or 475 to 625 nm. Thus, the composite film prevents the display screen from coloring.
The above polymer films (phase difference films) have been developed for the purpose of the elimination of coloring of the STN-LCD. Thus, the coloring is reduced and the display provided with the film shows almost a black-and-white image. Further, the liquid crystal display utilizing the FTN mode has the advantages of lightweight and low production cost compared with the NTN mode-display as mentioned previously.
As described above, the above liquid crystal display has been improved in elimination of coloring, so far as the coloring view from the direction vertical to the screen is concerned. However, when the liquid crystal display is viewed from an oblique direction, unfavorable viewing angle characteristics such as coloring and disappearance of displayed image are observed.
In order to improve the viewing angle characteristics, various phase difference films have been proposed. Their improvements are proposed from the viewpoint of changing refractive indices in three-dimension directions of the film. For instance, Japanese Patent Provisional Publication No. 2(1900).189518 discloses a phase difference film in which "n.sub.MD " is less than both n.sub.TD and n.sub.ND (i.e., phase difference film having a negative uniaxiality), wherein "n.sub.MD " is a refractive index in a stretched axis direction of uniaxially stretched polymer film (phase difference film), "n.sub.TD " is a refractive index in a direction intersected at right angles to the stretched axis of the film and "n.sub.ND " is a refractive index in a normal line direction to the surface of the film. The directions of these symbols are illustrated in FIG. 3 of the attached drawings.
Japanese Patent Provisional Publication No. 2(1990)-256023 discloses a phase difference film as described below. The film is a laminated film comprising a film A and a film B. The film A is at least one film having light transmission properties and at least one optic axis at an angle of not less than 45 degree to a normal line direction of the surface of the film, or satisfying the following formula: EQU n.sub.ND -(n.sub.MD +n.sub.TD)/2&gt;0
wherein n.sub.ND, n.sub.MD and n.sub.TD mean the above definitions. And the film B is at least one uniaxially stretched film of a polymer having a positive intrinsic birefringence and light transmission properties. The film A is, for example, a biaxially stretched (orientated) polystyrene or polyacrylate film having a negative intrinsic birefringence.
The above techniques using the phase difference film having a negative intrinsic birefringence are reasonable for enhancement of viewing angle characteristics. However, the film having a negative intrinsic birefringence is not satisfactory in productivity, yield and stability of retardation value during storage at high temperatures. In more detail, it is generally needed that the material (polymer) of the phase difference film satisfies the characteristics such as transparency, easiness of occurrence of birefringence by stretching, and sufficient flexibility and heat resistance so as not to damage the film in the procedures for its production such as film formation, stretching, transportation and winding.
Studies of the present inventors have revealed that among known phase difference films having a negative intrinsic birefringence a polystyrene film only has practical value in easiness of occurrence of birefringence. However, a phase difference film made of the polystyrene dose not exhibit a satisfactory heat-resistance and a good flexibility, so that the film is apt to be damaged in the process for the preparation.